As a device that controls a driving pressure of an actuator, a relief valve is known. This relief valve sets a maximum high pressure using a spring force of a spring and controls circuit pressure by communicating the circuit with a tank when a pressure equal to or more than the maximum high pressure is applied. As a component that changes the setting pressure, for example, as disclosed in JP1994-174122A, A device with an auxiliary piston disposed at the spring where pressure is applied to the auxiliary piston, the spring is bent, and an initial setting pressure is changeable is generally known.
On the other hand, in a construction machine, for example, as a device that controls a driving pressure of a rotating motor, a device disclosed in JP2011-017427A is known.
This type of device includes relief valves. The relief valves, which control driving pressure of the rotating motor, are connected in parallel with a pair of connecting passages. The pair of connecting passages allow the rotating motor to communicate with the hydraulic pump or the tank. The relief valves include an open/close valve at upstream. The relief valve couples a hydraulic motor for rotating an electric generator at downstream.
The setting pressure of the relief valve is set lower than that of a main relief valve that controls the maximum high pressure of the entire circuit.
If the rotating motor has a surplus driving pressure, the open/close valve is opened to guide the driving pressure of the rotating motor to the relief valve. Then, the relief valve is opened with the driving pressure of the rotating motor to guide residual oil of the rotating motor to the hydraulic motor.